Human bronchial explants, pulmonary alveolar macrophages (PAM) and blood monocytes metabolically activated benzo(a)pyrene (B(a)P) and 7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene (7,8-diol) to ultimate mutagens. Using a human tissue- and cell-mediated mammalian mutagenesis assay these activated metabolites caused increases of ouabain resistant (Or) mutation frequency and number of sister chromatid exchanges (SCE) in the cocultivated indicator cells (Chinese haster V-79 cells). Ouabain resistant mutation, SCE, nascent DNA synthesis and cytotoxicity were compared in V-79 cells treated with dihydrodiol epoxide derivatives of B(a)P. We found that inhibition of nascent DNA synthesis by these direct carcinogens was more closely related to cytotoxicity than to the induction of SCE or Or mutations. Caffeine inhibited nascent DNA synthesis and enhanced cell killing but did not influence the resulting induced levels of mutagenesis or SCE. Analysis of our data concerning the induction of SCE indicates that enhancement of SCE frequency by an agent, e.g., H2O2 and trans-PT(II) does not necessarily lead to increase in mutation frequency.